Flue gas desulfurization (FGD) occurs primarily in the flue gas absorber tower by means of spraying the flue gas with a mixture which causes the sulfur to precipitate out. This spraying is accomplished by installing spray headers across the tower opening directly in the flue gas path. In early systems, which were typically small, a single spray level was all that was needed because of the low gas flow rates. However, as systems became larger, the flue gas flow rates increased and the spray volume was increased up to pump capacity until additional spray levels were required.
The addition of spray levels is resisted because each level increases the absorber tower height by up to five feet or more and this can be critical in closely confined quarters. However, despite this reluctance, the ultimate need is dictated by the flue gas flow rate and the desire to remove as much sulfur as possible before releasing these gases to the atmosphere. Thus, it became quite common for a FGD system to incorporate five or more spray levels to accommodate the flue gas volume and sulfur density. Unfortunately, however, this caused the absorber tower to be raised in height about twenty feet or more with these multiple spray levels requiring a corresponding increase in pumping and piping requirements. Thus, to accommodate greater flue gas capacity, the tower height (as well as diameter) had to be increased which required more pumping power.
In addition to these multiple spray levels, there is also a need in some systems for a spare spray level which is to be utilized when one of the other levels is out of commission or being repaired. This only adds to the cost and height of the absorber tower.
In some cases, the FGD system incorporates a high liquid to gas (L/G) ratio as compared to the more common use of low ratios in other systems. The L/G ratio is a reference to liquid flow rate relative to gas volume whose units are gallon per minute per 1000 actual cubic feet per minute (gpm/macfm). This L/G ratio is determined by a number of process variables including reagent type, pH, solids particle size, reactivity, chloride ion concentration, sulfur dioxide concentration, sulfur dioxide removal, reagent utilization, spray drop size, etc. A high L/G implies a high liquid flow rate relative to gas volume which is typically a value of 40 to 140 or more. Consequently, a low L/G ratio would generally be a value of less than 20. One way of accomplishing a high L/G ratio is by simply adding more spray levels to the absorber tower so that more of the mixture can be sprayed onto the flue gas. This, however, does not eliminate the need for a taller tower nor does it diminish the need for more pumping power so that the mixture can be delivered at the higher elevation.
Since every foot of absorber tower height is costly, and since each bit of energy needed to deliver the mixture to its assigned elevation must be paid for, it is desirable to both cut the absorber tower height and reduce the energy requirements on each FGD system installed. It is thus an object of this invention to provide a system that both reduces absorber tower height and lowers energy costs. It is another object of this invention to provide an FGD system that can incorporate a spare spray without also increasing absorber tower height. These and other features of this invention are set out below.